A 40,000 mAh power bank usually yields about 120 Wh of usable energy, giving 9–12 phone charges or 3–6 laptop hours depending on draw.
You came here for straight, practical estimates. This guide explains how much runtime you can expect from a large pack, why numbers vary, and how to size choices for phones, tablets, game consoles, and notebooks in practice.
Quick Answer And The Variables That Matter
A pack labeled 40,000 mAh contains a stack of 3.6–3.7 V cells. Energy equals milliamp-hours times voltage divided by 1,000. That puts the raw energy near 148 watt-hours. Some of that never reaches your device because the pack boosts voltage, the cable drops a bit of power, and your gadget’s charger wastes a little more. Usable energy often lands between seventy-five and eighty-five percent of the raw number. For planning, budget about 120 Wh usable.
A low draw like a phone sipping at five to ten watts wastes less. A hungry notebook at sixty to one hundred watts runs the conversion hardware hotter and wastes more.
Runtime Estimates For Common Devices
Use the table below as a starting point. It assumes about 120 Wh usable and round figures for device batteries.
| Device Type | Typical Battery (Wh) | Approx. Charges/Hours |
|---|---|---|
| Smartphone | 12–18 | 9–6 full charges |
| Small Tablet | 25–30 | 4–3 full charges |
| Large Tablet | 35–45 | 3–2 full charges |
| Nintendo Switch/Handheld | 16 | 7–6 full charges |
| Mirrorless Camera | 14 | 8–7 full charges |
| Ultrabook At 20–30 W | — | 6–4 hours of use |
| Work Laptop At 45–65 W | — | 3–2 hours of use |
| Gaming Laptop At 90–120 W | — | 1.3–1 hour of use |
Close Variant: 40000mAh Bank Runtime Explained Step By Step
Step 1: Convert Milliamp-Hours To Watt-Hours
Use this equation: Wh = (mAh × V) ÷ 1000. For a pack built from 3.7 V cells, that is 40,000 × 3.7 ÷ 1000 ≈ 148 Wh. Packs that publish watt-hours already did this math.
If a label lists only five-volt numbers, that reflects output, not the native cell voltage. Do the math with the cell side, not the USB side, or your result will be inflated.
Step 2: Account For Conversion Losses
Voltage boosting, regulation, and charging overhead trim capacity. A fair planning number for a healthy pack is eighty to eighty-five percent. That places our usable energy near 118–126 Wh. Low power loads may see a little more. High power loads may see a little less.
Step 3: Relate Energy To Your Device
There are two easy paths. When you know your device’s battery in watt-hours, divide usable pack watt-hours by that number to get expected full charges. When you know the power draw in watts, divide usable pack watt-hours by the draw to get hours of runtime on external power.
Why Two Packs With The Same Label Perform Differently
Cell Quality And Actual Capacity
Not every pack hits its label. Better cells hold their voltage under load and keep more energy at the usable end. Lower grade cells sag sooner and the control board cuts power earlier.
Voltage And USB Mode
Packs that offer USB Power Delivery at twenty, twenty-eight, thirty-six, or forty-eight volts can feed notebooks more efficiently than five-volt only models. A notebook that requests a native higher voltage avoids extra conversion steps inside the laptop and runs longer.
Cables, Connectors, And Heat
Long or thin cables waste watts. Loose connectors spark tiny drops. Charging in a hot car makes the pack throttle. Small things add up over hours.
PD Profiles, Output Limits, And What They Mean For Runtime
Big capacity does not always equal big output. A pack with a 148 Wh rating might still cap at sixty or one hundred watts on a single port. If your notebook wants eighty-seven watts but the pack can only supply sixty, the notebook may run while idling yet drain during heavy work. When the draw dips back under the cap, the battery climbs again. Net runtime depends on that tug of war. Packs that offer the latest USB Power Delivery extended range profiles can deliver up to two hundred forty watts.
For an official view of the voltage and wattage menu that USB-C can advertise, see the USB Power Delivery overview.
Air Travel Limits For Large Packs
Energy rating also affects what you can bring on a plane. Around 148 Wh lands above the common 100 Wh threshold. That places it in the 101–160 Wh bracket where most airlines require approval and carry-on only. Packs above 160 Wh are not allowed. Always check the rating printed in watt-hours on the pack label, keep the terminals protected, and never stash spares in checked bags.
For current rules in the United States, review the FAA’s PackSafe lithium batteries page.
Make Your Estimate: A Simple Worksheet
Grab two numbers: usable pack watt-hours and either device battery watt-hours or device power draw in watts. Then use the matching row below.
| What You Know | Quick Math | What You Get |
|---|---|---|
| Device battery in Wh | Usable Wh ÷ device Wh | Full charges |
| Device draw in W | Usable Wh ÷ W | Hours of run time |
| Only mAh on a label | (mAh × 3.7) ÷ 1000 | Pack Wh (raw) |
Worked Examples You Can Copy
Phones
Assume a phone battery near 15 Wh. With 120 Wh usable, expect around eight full charges if the battery runs down fully each time. Real use mixes top-offs and standby drain, so the cycle count will look a bit lower over a weekend trip. Fast charge modes raise heat and losses a touch, trading a slice of efficiency for speed.
Tablets
A compact slate might sit near 28 Wh. That yields about four full charges. A big slate at 40 Wh drops to roughly three. If you watch streams with screen brightness up, treat the device more like a small laptop by using the power draw method instead of the battery size method.
Ultrabooks
On a light workload, many thin notebooks sip around 20–30 W. Divide 120 Wh by that draw and you land between four and six hours on pack power. If your system pulls 45–65 W while compiling code or editing footage, expect two to three hours.
Gaming Laptops
These machines spike to 100 W and past during play. External packs that top out at 100 W may only slow the internal battery drain. Pause the session, drop to a capped frame rate, or game while plugged into wall power to save the pack for lighter tasks.
How To Stretch Runtime From A Big Pack
Match The Voltage Profile
Use a USB-C cable rated for the wattage you need. Let a Power Delivery-capable pack feed a notebook at a higher voltage so the notebook’s own DC-DC converter does less work.
Charge Then Use
Charging a device while using it keeps both systems warm and wastes energy. Short charging stints, then device use, often nets more total screen time out of the same energy.
Mind Phantom Loads
Idle draw from dongles, receivers, and background processes add up. Disable high refresh displays, tone down brightness, and kill unneeded radios when you want long stints away from wall power.
Safety, Care, And Pack Health
Store near half charge if the pack will sit unused for a month or more. Keep it cool and dry. Avoid metal clutter near the ports. Retire a pack that swells, smells odd, or shuts off under light load. Airlines and regulators prohibit damaged batteries on planes for good reasons. At home, a metal tray or a tile shelf is a prudent place to charge.
Specs To Read Before You Buy
Published Watt-Hours
A clear watt-hour figure beats a lone milliamp-hour number. It proves the maker understands airline rules and energy math. A label near 148 Wh is the honest number for this class.
Per-Port And Total Output
Check both. A pack might advertise one hundred eighty watts total, split across two or three ports, with a single port capped at one hundred. If you need the full blast on one cable, confirm the single-port rating.
Pass-Through And UPS Behavior
Some packs can charge and discharge at once. That is handy at a desk, but heat rises and efficiency slips. For travel, a simple cycle of charge, then use, treats the pack better.
Common Myths, Debunked Quickly
Five-Volt Output Doesn’t Create More Energy
No. Energy lives on the cell side. The pack must boost to the USB voltage. Quoting output-side math without the cell voltage inflates claims.
Big Capacity Doesn’t Guarantee All-Day Laptop Power
Not so. A high draw machine can outrun a pack’s per-port cap. Look for Power Delivery extended range support and match the wattage.
Airlines Use Watt-Hours To Set Rules
Rules hinge on watt-hours. Around 148 Wh sits in an approval-required bracket for carry-on only. Check your carrier before you fly.
Bottom Line: Plan With Watt-Hours
The fastest way to predict runtime is to treat a big pack like a small tank of energy in watt-hours, then divide by either battery size or power draw. For a healthy 40,000 mAh unit, planning around 120 Wh usable sets honest expectations: many phone refills, several hours on a light notebook, and shorter stints on a power-hungry rig. With the right cable and PD match, you’ll squeeze more out of every charge safely.